PROPOSAL (Adapted from the applicant's abstract): IL-12 is one of the most potent antitumor agents, capable of eradicating immune-responsive tumors such as melanoma and renal cell cancer in mice more effectively even than IL-2. In human cancer patients treated with IL-12 on clinical trials over the past five years, IL-12 has had only modest activity as an antitumor drug. Several factors appear to underlie the low antitumor activity of IL-12 in humans. First, optimal immune activation by IL-12 requires the coordinated stimulation of lymphocytes with IL-12 plus either IL-15 or IL-2. Secondly, the investigators have found that, although IL-12 induces IL-15 production in vivo, it also has a tendency to rapidly dampen the lymphocyte response to IL-2 or IL-15 by downmodulating IL-2/IL-15 signaling. By interfering with important costimulatory signals from endogenous IL-2 or IL-15, IL-12 treatment paradoxically induces hyporesponsiveness of the immune system to IL-12. Lastly, they have recently discovered that some patients with melanoma or renal cell cancer have lymphocytes that lack the signaling molecules required for activation by IL-2 or IL-15, rendering them unresponsive to IL-12 treatment. In this application, they propose to pilot new trials of IL-12 in melanoma and renal cell cancer that will attempt to realize its full antitumor potential based on our current knowledge of these factors controlling in vivo immune activation by IL-12 in humans. These trials will use new dosing schedules of IL-12 to increase and sustain IL-12-induced immune activation, combine IL-12 with IL-2 or IL-15 to augment lymphocyte cytolytic activity and IFN-gamma production, administer IL-12 together with novel toxicity reduction agents, and use molecular screening to select for patients with intact cytokine signaling pathways that are most apt to respond to IL-12 therapy. An important component of these new clinical trials will be the correlative laboratory experiments performed using blood samples and tumor tissue derived from patients. These experiments will be pivotal in establishing, in humans, the role of IL-15 and other induced cytokines and chemokines such as IL-18 and IP-10 in the antitumor effect of IL-12, the ability of IL-12 to generate specific and non-specific antitumor immunity, the mechanism underlying the IL- 12-induced down regulation of lymphocyte IL-12 responsiveness and ways to prevent or reverse this, and the impact which acquired deficiencies in IL- 2/IL-15 signaling molecules (namely, Stat 1 and Stat 5) has on the effectiveness of IL-12 and IL-2.